Code modulation using narrow spectral notching

1. A method for encoding data in a signal using notch depth modulation, the method comprising notch filtering the signal at a preselected set of frequencies in accordance with data bits to be transmitted, wherein said step of notch filtering the signal comprises: determining a respective notch depth at each of the set of notch frequencies, the respective notch depths corresponding to the data bits to be transmitted in each respective notch frequency; and notch filtering the signal at each of the set of notch frequencies to the respective notch depth, wherein said step of determining a respective notch depth comprises selecting one of 2 N notch depths to represent a data symbol of the data, the data symbol representing N data bits.

2. The method of claim 1 , wherein notch filtering the signal further comprises determining the preselected set of notch frequencies.

3. The method of claim 2 , wherein determining the set of notch frequencies comprises pseudo randomly selecting the set of notch frequencies from a predetermined set of available notch frequencies.

4. The method of claim 3 , wherein selecting the set of notch frequencies comprises selecting the set of notch frequencies from a set of unevenly spaced available notch frequencies.

5. The method of claim 1 , wherein said step of determining a respective notch depth comprises selecting a true notch depth to represent a logic level of true for a data bit of the data and selecting a false notch depth to represent a logic level of false for a data bit of the data.

6. The method of claim 1 , wherein said step of determining a respective notch depth comprises selecting a true notch depth to represent a logic level of true for a data bit of the data and selecting a false notch depth of zero to represent a logic level of false for a data bit of the data.

7. The method of claim 1 , further comprising determining a respective notch width at each of the set of notch frequencies and notch filtering the signal at each of the set of notch frequencies to the respective notch width.

8. The method of claim 7 , wherein the signal is an audio signal, and wherein said step of determining a respective notch width comprises selecting a respective notch width at each of the set of notch frequencies based on a bit error rate.

9. A method for decoding data from a signal in which the data was encoded using notch depth modulation, the method comprising the steps of: determining notch spectral levels of the signal at each of a set of selected notch frequencies; determining notch spectral thresholds at each of the set of selected notch frequencies; and decoding the data by comparing the notch spectral levels with the corresponding notch spectral thresholds wherein said step of determining notch spectral thresholds comprises: determining between-notch spectral levels of the signal at frequencies between each of the set of selected notch frequencies; and determining the notch spectral thresholds at each of the set of selected notch frequencies based on the determined between-notch spectral levels.

10. The method of claim 9 , wherein said step of determining notch spectral thresholds comprises a step of tuning the notch spectral thresholds at each of the set of selected notch frequencies, said tuning comprising: step 1) adjusting each notch spectral threshold; step 2) decoding the data at each of the set of selected notch frequencies; step 3) determining the error rate for the decoded data; and repeating steps 1, 2 and 3 until a desired error rate is achieved for the decoded data.

11. The method of claim 9 , wherein said step of determining notch spectral thresholds comprises determining one notch spectral threshold at each of the set of selected notch frequencies, and wherein said step of decoding the data comprises comparing the notch spectral level with the corresponding notch spectral threshold at each of the set of selected notch frequencies.

12. The method of claim 11 , wherein said step of decoding the data further comprises decoding a true data bit at each of the set of selected notch frequencies where the notch spectral level is below the notch spectral threshold and decoding a false data bit at each of the set of selected notch frequencies where the notch spectral level is not below the notch spectral threshold.

13. The method of claim 9 , wherein said step of determining notch spectral thresholds comprises determining 2 N notch spectral thresholds corresponding to each of the set of selected notch frequencies, and wherein said step of decoding the data comprises determining data symbols at each of the set of selected notch frequencies, the data symbols each representing N bits of the data.

14. A decoding system for decoding data from a signal encoded using notch depth modulation, the system comprising: a digitizer which accepts the signal as input and outputs a digitized signal; a Fourier Transform processor coupled to said digitizer which converts the digitized signal to a set of spectra components; a threshold determiner coupled to said Fourier Transform processor which determines notch thresholds from the set of spectral components; and a decoder coupled to said Fourier Transform processor and said threshold determiner which decodes the data from the signal by comparing the set of spectral components to the notch thresholds, wherein said threshold determiner comprises a between-notch spectral level determiner which determines spectral levels at frequencies between selected notch frequencies.

15. The system of claim 14 , wherein said digitizer comprises: an analog band-pass filter to filter the signal; an automatic gain controller coupled to said analog band-pass filter which adjusts the amplitude of the filtered signal; and an A/D converter coupled to said automatic gain controller which converts the amplitude adjusted signal to the digital signal.

16. The system of claim 14 , wherein said decoder comprises: a data detector which detects raw data in the signal; and an error corrector coupled to said data detector which converts the raw data from the data detector into the decoded data.

17. The system of claim 16 , wherein said error corrector is further coupled to said threshold determiner to provide data error information to said threshold determiner.

18. In a broadcast system, a method for communicating inaudible data in an audio signal, the method comprising: notch filtering the audio signal at a set of notch frequencies to corresponding notch depths according to the data; communicating the notch filtered audio signal to a destination; and decoding the data from the notch filtered audio signal at the destination, wherein said step of notch filtering further comprises determining each respective notch depth and each respective notch width according to data error rate and inaudibility level.

19. The method of claim 18 , wherein said step of notch filtering comprises: determining the set of notch frequencies; determining a set of respective notch depths for each of the set of notch frequencies according to the data; and notch filtering the audio signal at each of the set of notch frequencies to according to the set of respective notch depths.

20. The method of claim 19 ,further comprising determining a respective notch width for each of the set of notch frequencies, and wherein said step of notch filtering the audio signal at each of the set of notch frequencies further comprises notch filtering the audio signal at each of the set of notch frequencies according to each respective notch width.

21. The method of claim 18 , wherein the inaudible data comprises broadcast verification data, the method further comprising monitoring the decoded data at the destination to monitor broadcast content.

22. The method of claim 18 , wherein the inaudible data comprises audience monitoring data, the method further comprising monitoring the decoded data at the destination to monitor audience broadcast receiving habits.

23. A method for encoding data in a signal using notch modulation, the method comprising the steps of: selecting at least one notch frequency in the signal; notch filtering the signal at notch frequency to produce a notch, wherein the depth of said notch is one of a plurality of available notch depths; wherein each of said plurality of notch depths represents a different logical state.

24. A method for encoding data in a signal using notch modulation, the method comprising the steps of: selecting at least one notch frequency in the signal; notch filtering the signal at notch frequency to produce a notch, wherein the depth of said notch is one of a plurality of available notch depths; wherein said notch filtering includes 2 N notch depths to represent an N-bit data symbol, wherein N is at least one.

25. A method for encoding a in a signal using notch modulation, the method comprising the steps of: selecting at least one notch frequency in the signal; notch filtering the signal at notch frequency to produce a notch, wherein the width of said notch is one of a plurality of available notch widths; wherein the width of said notch may be adjusted according to measured communication parameters.

26. A method for encoding data in a signal using notch modulation, the method comprising the steps of: selecting at least one notch frequency in the signal; notch filtering the signal at notch frequency to produce a notch, wherein the width of said notch is one of a plurality of available notch widths; wherein the width of said notch may be adjusted according to measured communication parameters; and wherein said communication parameters include at least one of inaudibility, signal noise levels, and data error rates.

27. A method for encoding data in a signal using notch modulation, the method comprising the steps of: selecting at least one notch frequency in the signal; notch filtering the signal at notch frequency to produce a notch, wherein the width of said notch is one of a plurality of available notch widths; wherein the width of said notch is adjustable in real time.